Improve exception on creating KClass for classes which are not classes in Kotlin

core/reflection.jvm/src/kotlin/reflect/jvm/internal/KClassImpl.kt

@@ -22,6 +22,8 @@ import org.jetbrains.kotlin.incremental.components.NoLookupLocation
 import org.jetbrains.kotlin.load.java.JvmAbi
 import org.jetbrains.kotlin.load.java.reflect.tryLoadClass
 import org.jetbrains.kotlin.load.java.structure.reflect.safeClassLoader
+import org.jetbrains.kotlin.load.kotlin.header.KotlinClassHeader
+import org.jetbrains.kotlin.load.kotlin.reflect.ReflectKotlinClass
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.platform.JavaToKotlinClassMap
@@ -41,7 +43,7 @@ internal class KClassImpl<T : Any>(override val jClass: Class<T>) : KDeclaration
                 if (classId.isLocal) moduleData.localClassResolver.resolveLocalClass(classId)
                 else moduleData.module.findClassAcrossModuleDependencies(classId)
 
-        descriptor ?: throw KotlinReflectionInternalError("Class not resolved: $jClass")
+        descriptor ?: reportUnresolvedClass()
     }
 
     val descriptor: ClassDescriptor
@@ -160,4 +162,31 @@ internal class KClassImpl<T : Any>(override val jClass: Class<T>) : KDeclaration
             packagePrefix + classSuffix
         }
     }
+
+    private fun reportUnresolvedClass(): Nothing {
+        val kind = ReflectKotlinClass.create(jClass)?.classHeader?.kind
+        when (kind) {
+            KotlinClassHeader.Kind.FILE_FACADE, KotlinClassHeader.Kind.MULTIFILE_CLASS, KotlinClassHeader.Kind.MULTIFILE_CLASS_PART -> {
+                throw UnsupportedOperationException(
+                        "Packages and file facades are not yet supported in Kotlin reflection. " +
+                        "Meanwhile please use Java reflection to inspect this class: $jClass"
+                )
+            }
+            KotlinClassHeader.Kind.SYNTHETIC_CLASS -> {
+                throw UnsupportedOperationException(
+                        "This class is an internal synthetic class generated by the Kotlin compiler, such as an anonymous class " +
+                        "for a lambda, a SAM wrapper, a callable reference, etc. It's not a Kotlin class or interface, so the reflection " +
+                        "library has no idea what declarations does it have. Please use Java reflection to inspect this class: $jClass"
+                )
+            }
+            KotlinClassHeader.Kind.UNKNOWN -> {
+                // Should not happen since ABI-related exception must have happened earlier
+                throw KotlinReflectionInternalError("Unknown class: $jClass (kind = $kind)")
+            }
+            KotlinClassHeader.Kind.CLASS, null -> {
+                // Should not happen since a proper Kotlin- or Java-class must have been resolved
+                throw KotlinReflectionInternalError("Unresolved class: $jClass")
+            }
+        }
+    }
 }